The study has 2 stages: first stage would be survey of 400 employees, who will fill out a questionnaire to identify occupational and demographic risk factors, a nutritional questionnaire, and data from periodic checkups will be collected. Participants will sign a consent form for vitamin D level determinations, and for freezing blood samples. From this survey, the participants that have vitamin D deficiency (25(OH)D levels < 20 ng/ml) will continue to the second stage of the study - administration of vitamin D or placebo. Participants in the interventional study will sign an additional consent form. Length of follow-up: one year. All parameters will be tested at 0, 6, and 12 months.